1. Field of the Invention
The present invention relates to the field of footwear. The invention concerns, more particularly, a sole structure for an article of footwear having a suspended footbed with a slatted structure that includes a plurality of beams for supporting a foot. The invention has application to a variety of footwear styles, including athletic footwear utilized for walking, running, or a plurality of other athletic activities.
2. Description of Background Art
Conventional articles of athletic footwear include two primary elements, an upper and a sole structure. The upper is often formed of leather, synthetic materials, or a combination thereof and comfortably secures the footwear to the foot, while providing ventilation and protection from the elements. The sole structure generally incorporates multiple layers that are conventionally referred to as an insole, a midsole, and an outsole. The insole is a thin, cushioning member located within the upper and adjacent the plantar (lower) surface of the foot to enhance footwear comfort. The midsole, which is traditionally attached to the upper along the entire length of the upper, forms the middle layer of the sole structure and serves a variety of purposes that include controlling potentially harmful foot motions, such as over pronation, attenuating ground reaction forces, and absorbing energy. In order to achieve these purposes, the midsole may have a variety of configurations, as discussed in greater detail below. The outsole forms the ground-contacting element of footwear and is usually fashioned from a durable, wear-resistant material that includes texturing to improve traction.
The primary element of a conventional midsole is a resilient, polymer foam material, such as polyurethane or ethylvinylacetate, that extends throughout the length of the footwear. The properties of the polymer foam material in the midsole are dependent upon factors that include the dimensional configuration of the midsole and the specific characteristics of the material selected for the polymer foam, including the density of the polymer foam material. By varying these factors throughout the midsole, the relative stiffness, degree of ground reaction force attenuation, and energy absorption properties may be altered to meet the specific demands of the activity for which the footwear is intended to be used.
In addition to polymer foam materials, conventional midsoles may include, for example, stability devices that resist over-pronation and moderators that distribute ground reaction forces. The use of polymer foam materials in athletic footwear midsoles, while providing protection against ground reaction forces, may introduce instability that contributes to a tendency for over-pronation. Pronation is the inward roll of the foot while in contact with the ground. Although pronation is normal, it may be a potential source of foot and leg injury, particularly if it is excessive. Stability devices are often incorporated into the polymer foam material of the midsoles to control the degree of pronation in the foot. Examples of stability devices are found in U.S. Pat. No. 4,255,877 to Bowerman; U.S. Pat. No. 4,287,675 to Norton et al.; U.S. Pat. No. 4,288,929 to Norton et al.; U.S. Pat. No. 4,354,318 to Frederick et al.; U.S. Pat. No. 4,364,188 to Turner et al.; U.S. Pat. No. 4,364,189 to Bates; and U.S. Pat. No. 5,247,742 to Kilgore et al. In addition to stability devices, conventional midsoles may include fluid-filled bladders, as disclosed in U.S. Pat. Nos. 4,183,156 and 4,219,945 to Marion F. Rudy, for example.
Despite the variations in midsole configurations and the various stability devices and fluid-filled bladders, conventional midsoles are primarily formed of a unitary element of polymer foam material. Polymer foam materials are often impermeable to air and liquids and are, therefore, relatively difficult to ventilate. In addition, polymer foam materials that provide a suitable degree of stability, ground reaction force attenuation, and energy absorption may be relatively inflexible and heavy. When midsoles are formed of lightweight polymer foams to increase flexibility and reduce weight, the polymer foam is susceptible to compression set. That is, the individuals cells within the polymer foam material may break down following repeated compressions. Furthermore, lightweight polymer foam materials may exhibit reduced stability in comparison with heavier, more dense polymer foam materials.